Temporal Coincidence between the Excitation of Ventromedial Hypothalamic Efferents and the Induction of Lordosis Reflex in Ovariectomized Estrogen-Primed Rats.

Abstract

Abstract. The temporal changes in excitability in ventromedial hypothalamic neurons which project to the midbrain central gray caused by chronic estrogen exposure were measured in ovariectomized rats. In 66 urethane-anesthetized animals which had either no capsule, or had a capsule containing estrogen implanted for 24 h, 48 h, 72 h or 96 h, 189 antidromic action potentials were recorded in the ventromedial hypothalamus (VMN) in response to stimulation of the midbrain central gray (CG). The antidromic activation threshold was lower in 40 recordings made from the VMN in the 72-h or 96-h group than in 67 from the no-treatment, 24-h or 48-h group. The decrease in the threshold coincided with the emergence of the lordosis reflex. In a separate cohort of ovariectomized rats, any absence of estrogen for 2-48 h before and during the measuring abolished the neuronal as well as the behavioral effects at 72 h. The results suggest that the lordosis reflex induced by estrogen alone depends on the excitation of VMN efferents to the CG. In addition, the indispensability of estrogen at the time of electrophysiological and behavioral evaluation indicates that a certain direct neural action of estrogen is involved in the behavioral activation. Implants of minute amounts of crystalline estradiol showed that the ventromedial nucleus of the hypothalamus (VMN) is the principal site for estrogen action to facilitate the lordosis reflex [9]. Stimulation and lesion studies have shown that the VMN facilitates the lordosis reflex via its descending projection to the midbrain central gray (CG) [10][11][12]. Estrogen-induced excitation of VMN neurons with axons to the CG in ovariectomized rats [13][14][15] corroborates the behavioral results. Experiments on neonatally gonadectomized males and neonatally androgenized females, in which the neural effects of estrogen in parallel with its behavioral capability to induce the lordosis reflex [16], provide additional evidence for the association of the neural effect of estrogen with facilitation of the lordosis reflex.In the present study we attempted to establish a

Estrogen‐induced dendritic spine elimination on female rat ventromedial hypothalamic neurons that project to the periaqueductal gray

“…Additionally, estrogen treatment lowers the stimulation threshold for antidromic activation (Sakuma and Pfaff, 1982;Sakuma and Akaishi, 1987) and the absolute refractory period (Akaishi and Sakuma, 1986) of VMH neurons that project to the PAG. These estrogen-induced increases in VMH projection neuron excitability have been shown to coincide temporally with the appearance of lordosis behavior (Tashiro et al, 1998). Although these results implicate vlVMH neurons projecting to the PAG in female sexual behavior, how a selective decrease in the spine density of a subset of these neurons contributes to the priming or execution of the behavior is unknown.…”

The Medial Amygdala Controls the Coital Access of Female Rats: A Possible Involvement of Emotional Responsiveness

“…Open field behavior and passive avoidance learning were tested 3 weeks after the end of tests for sexual behavior, in the absence of circulating sex hormones [41]. compartments of similar size, with walls painted either white or black.…”